i2c: imx-lpi2c: avoid taking clk_prepare mutex in PM callbacks

This is unsafe, as the runtime PM callbacks are called from the PM
workqueue, so this may deadlock when handling an i2c attached clock,
which may already hold the clk_prepare mutex from another context.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>